There are various known techniques for exploring the structure and/or composition of regions below the surface of the earth, including below the sea bed. Some of these techniques are “active” in that they require the use of a specific controlled source of energy to probe the region and then measure the effect of the region on the transmitted energy. A typical example of such an active technique is the well-known seismic exploration technique, in which one or more sources produce impulsive or swept-frequency pressure waves which penetrate the earth and the returned pressure variations are measured by geophones or hydrophones.
Other techniques are of the “passive” type in that they do not require energy to be produced by a source but, instead, make use of already-existing energy to provide information about the structure or composition below the surface of the earth. One such technique is known as hydrocarbon microtremor analysis (Hymas), for example as disclosed in Dangel et al, 2001, “Phenomenology of tremor-like signals observed over hydrocarbon reservoirs (just related and higher frequencies)”. Hymas attempts to measure a continuous signal or tremor in a frequency range between 1 and 10 Hz over the reservoir (not “outside” or “beside” it). As illustrated in FIG. 1 of the accompanying drawings, sensors such as 1 and 2 embedded in the surface 3 of the earth measure such tremors. The sensors 1, 2 are in the form of extremely sensitive seismometers having a sufficiently broad bandwidth to be sensitive to the frequency range of interest. The seismometers are typically arranged as a two-dimensional array or grid with the spacing between seismometers typically being from a few hundred meters to a kilometer, depending on the spatial resolution required. The outputs of the seismometers are recorded for a period of typically 24 hours. It is not necessary for the recordings to be made simultaneously and fewer seismometers may be used by changing their locations after each recording period.
This technique is based on the assumption that a hydrocarbon reservoir 4 interacts with ambient seismic waves to modify the waves in a measurable way. FIGS. 2 and 3 of the accompanying drawings illustrate this. The seismometer 1 is located such that there is no hydrocarbon reservoir in the region of the earth below it. FIG. 2 illustrates the result of analysing the recorded data from the sensor over a recording period of typically 24 hours. The resulting data are plotted as a frequency spectrum of amplitude in arbitrary units against frequency in Hertz (Hz). The seismometer 2 is located above the hydrocarbon reservoir 4 and the spectrum obtained from the recorded data is shown in FIG. 3. It is believed that the presence of significantly increased energy in the frequency band around 3 Hz results from the modified seismic waves 5 produced by the hydrocarbon reservoir 4 and so provides a direct indication of the presence of the hydrocarbon reservoir.
A problem with the Hymas technique is that it is based on the detection and recording of very weak seismic tremor signals. However, human activity provides relatively high amplitude noise in the frequency range investigated in the Hymas technique. For example, noise resulting from vehicles, pumps and drilling occurs in this frequency range and may make the Hymas technique unusable in many situations.
Another known passive technique is referred to as ambient noise tomography (ANT). This technique is based on the use of conventional seismometers which have been recording very low frequency ambient noise for very long periods of time, for example of the order of years. The frequency range of interest is below 0.5 Hz and mainly below 0.05 Hz. Although the seismometers may be spaced apart by hundreds of meters for special cases, they are typically spaced apart by hundreds of kilometers and even globally around the earth. It is necessary for recordings to be made simultaneously and this reveals subsurface information between the locations of pairs of seismometers. An example of this technique is disclosed in Bensen et al 2007, “Processing seismic ambient noise data to obtain reliable broad-band surface wave dispersion measurements”.
The data from such seismometer arrangements are processed so as to extract the Green Function by cross-correlation of two simultaneous recordings. The results for each frequency are then used subjected to a dispersion analysis followed by a tomographic inversion so as to obtain a laminar image. The resolution is such as to provide structural information about the earth on the continental scale. Thus, this technique is generally not of particular interest when looking at smaller regions, for example, when exploring for or monitoring hydrocarbons, because of the insufficient spatial resolution.
Russian Patent no. 2271554C1 discloses a technique for recording and analyzing ambient noise in a frequency range below 15 Hz. In a first phase of this technique, ambient noise is simultaneously recorded (“synchronised”) by means of at least three sensors which are sensitive to the vertical component of the ambient noise signal. The data are then analyzed to estimate dispersion curves within the area being investigated.
In the next phase, ambient noise is recorded using at least two synchronised sensors sensitive to the vertical component. One of the sensors is used as a permanent station at the center of the area being surveyed during the whole of the survey. The or each other sensor is moved to different locations within the area being surveyed so as to record the vertical component of the ambient noise at each of the locations for an unspecified time period. The recordings obtained by the non-fixed sensors are then calibrated by calculating the differences between the power spectra of the permanently located sensor and the “moving” sensors. From the estimated dispersion curve, a depth is calculated, in an unspecified way, for each frequency. Maps are then drawn for each frequency or depth of the power spectra differences. Because the sensors are sensitive only to the vertical component of the measured ambient noise, this technique is limited to Rayleigh waves.